This invention relates to silicon wafer electroplating and mass spectrometry analysis technology. More specifically, it relates to analysis of electroplating bath constituents during integrated circuit fabrication. Even more specifically, the invention pertains to a particular monitoring and feedback system used for analysis and control of electroplating bath formulations.
Improved integrated circuit fabrication processes continue to necessitate more complex and demanding control of process parameters to ensure wafer uniformity and quality. Electroplating is a good example. Electroplating for integrated circuit fabrication is typically performed in a series of plating steps, with each having a particular hardware configuration and specific plating bath formulation. Often bath formulations include not only metal salts, but also acids, bases, organic additives and the like. More than ever, it is critical to monitor plating bath electrolyte constituents and maintain bath formulations within a specific range of parameters to ensure the desired outcome and quality of a particular plating process.
Conventional methods of assaying bath constituents commonly employ cyclic voltammetric stripping (CVS) or other forms of Faradaic electroanalysis, which have limitations in specificity and sensitivity. For example, voltammetric analyses suffer from lack of detection capability for compounds and ions that are not electrochemically active over the range of potentials used Also, many of the organic additives commonly used in some electroplating scenarios to alter the rate of metal deposition can not be detected using such techniques. In such cases, a true xe2x80x9cpicturexe2x80x9d of plating bath chemistry is not obtained using voltammetric analyses. Additionally, voltammetric analyses are sensitivity-limited by matrix effects (convoluted electrochemical interactions due to the response of breakdown products).
High-pressure liquid chromatography (HPLC) has been proposed as a method to monitor plating bath constituents by Taylor et al. xe2x80x9cElectroplating Bath Control for Copper Interconnects,xe2x80x9d Solid State Technology, vol. 4, issue Nov. 11, 1998. In this article, the authors describe using HPLC to separate electrolyte species. Although HPLC techniques have improved dramatically over the past decade, this type of analysis has limitations with regard to plating bath composition as well. While organic additives are well suited for chromatographic separation, metals, metal salts, and important ionic bath species are not. Analysis of purified bath components via chromatography can provide valuable information about plating bath chemistry, but, a more complete xe2x80x9cpicturexe2x80x9d of the bath chemistry is obtained only from analysis of xe2x80x9cintactxe2x80x9d plating bath samples. Also HPLC techniques tend to use large amounts of solvent, which is of environmental concern and creates costs associated with waste disposal.
Thus, while CVS and HPLC techniques are complementary with regard to which plating bath constituents can be analyzed, neither analysis giving information on all bath components.
Another problem associated with conventional plating bath analysis is time, or more specifically turnaround. Although voltammetric analysis and HPLC techniques have improved to include shorter analysis time frames, the time necessary for these analyses as compared to the time frame of possible change in a plating bath composition can be inadequate. Under such conditions, data regarding composition change obtained from plating bath analysis is rendered useless because the data may no longer reflect the actual bath formulation. This can be particularly problematic when such data is used to adjust bath component stoichiometries, i.e. the stoichiometry imbalance noted in the analysis can be compounded by addition of bath components based on inaccurate data.
What is needed therefore is improved technology for analysis and control of electroplating bath formulations during electroplating and electroplating processes during integrated circuit fabrication.
The present invention provides methods and apparatus for monitoring and controlling electrolyte bath composition. The invention also provides control of electroplating processes based on plating bath composition data. The invention accomplishes this by incorporating mass spectral analysis into a feedback control mechanism for electroplating. Mass spectrometry is used to identify plating bath conditions and based on the results, the plating bath formulation and plating process are controlled.
One aspect of the invention pertains to a method of using atmospheric pressure ionization mass spectrometry, API-MS. Mass spectrometry is particularly well suited to analysis of plating bath constituents because it does not share many of the limitations of the aforementioned methods. For example, mass spectral analysis is not limited by matrix effects. Also, sample volumes necessary to carry out a mass spectral analysis are typically very small. Turnaround is very short, making close-loop process control via constant bath analysis feasible. Additionally, mass spectrometry can detect molecules not easily ionized by using post-bath addition of ionization enhancers. Mass spectral analysis provides a more complete xe2x80x9cpicturexe2x80x9d of plating bath chemistry, because organic additives, metals, and their salts are easily detected. Thus, electroplating chemistries, mechanisms, and kinetics can be better characterized. Finally, by using an atmospheric pressure ionization source, plating baths can be easily sampled without undue modification of existing plating apparatus.
Of course other MS techniques can be employed in the method. Suitable techniques would include Ion Trap Quadrupole (single and triple quad), Magnetic Sector, Time-Of-Flight (TOF), Fourier-Transform MS (FT-MS), and the like.
Another aspect of this invention pertains to methods of controlling an electroplating process. These methods may be characterized by the following elements: (a) obtaining a sample of electrolyte from the electroplating process; (b) analyzing the sample of electrolyte by mass spectrometry to obtain a mass spectral result; (c) comparing the mass spectral result to a plurality of known mass spectral results; and (d) adjusting conditions of the electroplating process in response to the comparison. Electrolyte sampling may be done directly from a plating bath or from a separate vessel that serves as a collector of a small quantity of electrolyte.
Another aspect of this invention pertains to the logic associated with using mass spectral data for feedback control of an electroplating process. Preferably data from a mass spectral analysis is stored in a memory device. Then the data is compared to a data set of known mass spectral results (provided from plating bath compositions comprising organic plating additives, breakdown products of said additives, metals, metal ions, counter ions, and the like). Thus, a library search method is used as an element in the feedback control invention. The comparison involves comprises determining whether the data from the mass spectral analysis falls within a specified tolerance of a target result that is one of the data set of known mass spectral results. From the comparison, the logic determines commands for controlling the electroplating process. As mentioned, the invention finds particular use in the context of copper electroplating. Copper electroplating during damascene processing is becoming increasingly important and complex. The logic of the invention provides an efficient method of monitoring and controlling individual plating bath (many at once) chemistry and hardware during electroplating. This allows for improvement in throughput and wafer uniformity.
Yet another aspect of this invention pertains to apparatus for controlling an electroplating process. In certain embodiments the apparatus may be characterized by the following elements: (a) a mass spectrometer equipped with an electrolyte sampling device and an ionization source configured to deliver a sample of ionized electrolyte to the mass spectrometer, and (b) an associated logic for controlling the electroplating process based on a plurality of data obtained from the mass spectrometer upon analysis of the sample of ionized electrolyte. Preferably, the control element comes in the form of commands sent to plating hardware by the logic as a result of a comparison of the plurality of data obtained from the mass spectrometer with a known plurality of mass spectral data. The associated logic of the apparatus controls plating hardware through adjustment, for example valves for introducing plating bath constituents and formulations, electric field shaping apparatus, current flow, voltage levels, wafer handling apparatus, and electrode movement apparatus. Apparatus of the invention may sample electrolyte directly from a plating bath or indirectly, by use of an intermediary sample collection vessel. In many cases, individual components of the apparatus can be purchased commercially. Their configuration and programming constitute novelty in this case. The associated logic may be implemented in any suitable manner. Often it will be implemented in computer hardware and associated software for controlling the operation of the computer.
Further, the method and apparatus of the invention can be used for feedback control for other wet processes for silicon wafers in integrated circuit fabrication in which wafers are processed in a solution. For example, electroplanarization and wet etching (such as buffered oxide etch or BOE). In such wafer wet processes, API-MS in particular is well suited as an analytical tool for providing feedback control.